Several vehicle control systems, which are used to augment the driving capability of a vehicle operator, currently exist. Those control systems include anti-brake-lock system (ABS), traction control system (TCS), and stability controls. Example stability control systems are electronic stability control (ESC) systems or sometimes referred to as yaw stability control (YSC) systems. Systems of this kind are also sometimes called ESP (Electronic Stability Program) systems or DSTC (Dynamic Stability Traction Control) systems.
The stability control systems are utilized to maintain controlled and stable vehicle operations for improved vehicle and occupant safety. The stability control systems are often used to maintain control of a vehicle following a desired travel direction, to prevent the vehicle from spinning out and help the driver maintain directional stability when cornering. This function is enabled through the ABS system to brake one or more of the wheels if a lateral slide or skidding is detected. More specifically, the above yaw stability control systems typically compare the desired direction of a vehicle based upon the steering wheel angle and the path of travel, which is determined from motion sensors located on the vehicle. By regulating the amount of braking at each corner of the vehicle and the traction force of the vehicle, the desired path of travel may be maintained.
Existing stability control systems are designed to correct undesired vehicle motion caused by a tire force disturbance, such as a tire force difference due to a road surface disturbance or due to a mismatch between the driving intention of a driver and a road surface condition. This mismatch usually happens when there is a significant difference between the front and the rear tire lateral forces applied to the vehicle (referred to as the lateral tire force difference), or there is a significant difference between the right and the left tire longitudinal tire forces (referred to as the longitudinal tire force difference), or a combination thereof. Such a tire force difference is called a tire force disturbance.
The existing yaw stability control systems are effective in controlling the undesired vehicle motions due to the aforementioned tire force disturbance. The yaw stability control systems activate brakes, reduce engine torque, or vary the driving torque at individual wheels or axles so as to generate an active tire force difference to counteract the effect of the tire force disturbance. That is, the control mechanism and the vehicle disturbance are from the same source: the tire force variations or the tire force differences.
For example, when a vehicle is driven at a high speed to negotiate a turn, the vehicle could saturate its front tire cornering forces such that there is a front-to-rear tire lateral force difference. Such a tire force disturbance will generate a yaw moment disturbance, which causes the vehicle to steer less than that requested by the driver. This is referred to as an understeer situation. When the existing yaw stability control systems are used, the rear inside wheel is braked to add a longitudinal force to generate a yaw moment to counteract the yaw moment disturbance generated by the tire force disturbance due to the front-to-rear tire lateral force difference.
However, this brake intervention is usually performed as a function of yaw rate error, where the yaw rate error is determined as the difference between a yaw rate target and a sensed yaw rate. The yaw rate target is normally calculated from a steering wheel angle, which may be considered indicative of the driver intent, and the vehicle velocity using a nonlinear, so called, bicycle model. This bicycle model is nonlinear in terms of a tire to road friction compensation.
Experienced drivers are sensitive to the torque feedback in the steering wheel. The steering wheel torque will decrease, even for higher steering wheel angles, before the position where the peak friction is passed. Using this information, provided through the steering wheel, the experienced driver may perform compensatory steering in order to utilize the tire to road friction in an optimal way on the front axle.
However, such driving by an experienced driver will result in small yaw rate errors in the yaw stability control system, usually below a threshold of the yaw stability control system, wherefore no yaw stability control intervention will be performed by the yaw stability control system. As a consequence thereof the rear axle tire to road friction of the vehicle will not be used in an optimal way.